1. Field of the Invention
The present invention relates to a memory device. More particularly, the present invention relates to a flash memory device and a fabrication method thereof.
2. Description of Related Art
Electrically erasable programmable read only memory (EEPROM) is a type of non-volatile memory that allows multiple data reading, writing and erasing operations. In addition, the stored data will be retained even after power to the device is removed. With these advantages, electrically erasable programmable read only memories have been broadly applied in personal computers and electronic equipment.
A floating gate and a control gate of a typical flash memory device are fabricated with doped polysilicon. The floating gate is isolated from the control gate with an inter-gate dielectric layer, and is isolated from the substrate with a tunneling dielectric layer. The tunneling dielectric layer, the floating gate, the inter-gate dielectric layer and the control gate constitute a stacked-gate flash memory device. During a writing/erasing operation by a flash memory device, bias voltages are applied to the control gate, the source region and the drain region to inject electrons into the floating gate or to eject electrons from the floating gate. During a reading operation by a flash memory device, an operating voltage is applied to the control gate. The charge carrying state of the floating gate then affects the opening/closing of the channel underneath, in which the stored data as being [0] or [1] is determined.
When the gate couple ratio (GCR) between the control gate and the floating gate becomes higher, a lower operating voltage is required for operating the memory device. Therefore, the current industry is focused on increasing the capacitance of the inter-gate dielectric layer in order to raise the gate couple ratio. Generally speaking, increasing the capacitance of the gate dielectric layer includes increasing the area between the control gate and the floating gate. However, due to the limitations inherent in the manufacturing process, for example, the patterning process for the control gate and the floating, to further increase the area between the control gate and the floating gate is restricted. Consequently, to increase the level of integration of the device is thereby hindered.